1. Field of the Invention
The present invention relates to a voltage non-linear resistor comprising, as its main ingredient, zinc oxides, and more particularly a voltage non-linear resistor which is excellent in varistor voltage (VlmA) characteristics, lightning discharge current withstanding capability and life performance against applied voltage, and exhibits a strong coherency between its disc-like resistance element and insulating covering layer, and also to a process for manufacturing the same.
2. Description of the Prior Art
As a manufacturing process of voltage non-linear resistors having been heretofore extensively utilized in voltage stabilizing devices, surge absorbers, arrestors, etc. which have characteristics of acting as an insulator usually but as a conductor when an overcurrent flows, there is widely known, for example, a process for manufacturing a voltage non-linear resistor by forming a disc-like body from a starting material mixture consisting of 0.1-3.0% Bi.sub.2 O.sub.3, 0.1-3.0% Co.sub.2 O.sub.3, 0.1-3.0% MnO.sub.2, 0.1-3.0% Sb.sub.2 O.sub.3, 0.05-1.5% Cr.sub.2 O.sub.3, 0.1-3.0% NiO, 0.1-10.0% SiO.sub.2, 0.0005-0.025% Al.sub.2 O.sub.3, 0.005-0.3% B.sub.2 O.sub.3 and the remainder of ZnO (% stands for mole %) and then sintering the formed body.
Many attempts have been made to improve various performances of voltage non-linear resistors obtained according to the conventional process, such that, as measures for humidity proof and flashover prevention, a high resistance layer comprising an epoxy resin, etc. is provided on a peripheral surface of a disc-like resistance element or, in order to attain a minification by increasing the varistor voltage, the SiO.sub.2 content in the element is increased or a sintering temperature is lowered.
Conventional voltage non-linear resistors manufactured by the above-mentioned process have a wide composition range of components which causes a low cohering strength between the resistance element and the high resistance layers on its peripheral side surface and said cohering strength further decreases with lowering of the sintering temperature, so that flashover of the element has been unable to be effectively prevented. Consequently, a voltage non-linear resistor having a varistor voltage of 400 V/mm or more and being satisfactory in lightning discharge current withstanding capability and ife performance against applied voltage which are particularly important in protection of an electrical insulator, has not been obtainable.